Heat-sensitive recording materials comprising an electron donating colorless dye (color former) and an electron accepting compound (color developer) have been disclosed in Japanese Patent Publications Nos. 14039/70 (corresponding to U.S. Pat. No. 3,539,375) and 4160/68 (corresponding to U.S. Patent Application Ser. No. 512,546). Such heat-sensitive recording materials must have at least the following characteristics: (1) the color density and the color sensitivity must be sufficiently high, (2) fogging (coloration during preservation before using) is not caused and (3) the colors developed have sufficient fastness. However, present recording materials do not completely satisfy these requirements.
Recently, studies concerning the above described requirement (1) have been carried out due to recently developed high speed heat-sensitive recording systems.
In order to meet requirement (1) one approach has been to elevate the melting point of the electron accepting compound from 60.degree. C. to 100.degree. C. However, in phenolic compounds which have been widely used at present as the electron accepting compounds, it is difficult to control the melting point. Further, the phenolic compounds have little practical value, because they are expensive.
Other approaches have been described in Japanese Patent Publications Nos. 17748/74 and 39567/76. These approaches involve the use of a combination of an organic acid and a phenolic compound as an electron accepting material or the use of polyvalent metal salts of compounds having an alcoholic hydroxy group. Further, it has been described in Japanese Patent Publication No. 29945/76 to use hydroxyethyl cellulose and a copolymer of maleic acid anhydride slat.
Further, the addition of waxes has been described in Japanese Patent Publication No. 27599/76 and Japanese Patent Application (OPI) No. 19231/73 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".).
Additional approaches have been described in Japanese Patent Applications (OPI) Nos. 34842/74, 115554/74, 149353/75, 106746/77, 5636/78, 11036/78, 48751/78 and 72996/81. The approaches involve adding nitrogen containing organic compounds such as thioacetanilide, phthalonitrile, acetamide, di-.beta.-naphthyl-p-phenylenediamine, aliphatic acid amide, acetoacetanilide, diphenylamine, benzamide or carbazole, etc., thermoplastic substances such as 2,3-m-tolylbutane or 4,4'-dimethyl-biphenyl, etc. or carboxylic acid esters such as dimethyl isophthalate, diphenyl phthalate or dimethyl terephthalate, etc. as a sensitizer.
However, the heat-sensitive recording materials produced by the above-described techniques are insufficient with respect to color density and the color sensitivity.